When I took French in school, IIRC I learned that you use the definite article when talking about an object in general. For example, to say I like apples, you would say J'aime les pommes. In this case, Le garcon mange les pommes could mean that the boy eats apples in general, or he could be eating specific apples (THE apples). @SnowboardNW below mentions context, which would help in determining the intended meaning of this sentence.

Native English speaker and teacher. The boy eats the apples is not correct. The boy eats apples. The is used as a reference to something specific. The apples would mean... you know, the apples we bought earlier. Since there is no context, the cannot be used.